In the expanding Universe, the number densities of all particles decrease
with time, so that the magnitude of the baryon density (or that of any
other particle) has no meaning without also specifying when it is
measured.
To quantify the universal abundance of baryons, it is best to compare
nB to the CBR photon density
n. The ratio,
nB /
n is very small, so that it is convenient to
define a quantity of order unity,

(7)

where
B is the
ratio (at present) of the baryon density to the critical
density and h is the present value of the Hubble parameter in
units of 100 km s-1 Mpc-1
(BBh2).